It may very well be that we are talking at cross purposes here but in relation to your point number 2. I did a very quick and dirty test on my 5diii shooting in burst mode for 10 seconds firstly with the image only being written to the CF card and secondly with the image being written to both cards. In the first test I got 32 RAW files saved and in the second test I got just 20.

This is why I only have the camera set to record to one card at a time as this is more important to me than maybe losing images due to the CF card having a problem.

Am I missing something here?

No, I don't think you are missing anything here. Your results seem consistent with how I understand the camera to work.

My testing on my 5D3 was slightly different from yours. I first set the camera to record the same image type (raw in my case) to both cards and then used burst mode until the buffer filled up (I could tell because the shutter clicks suddenly slowed down dramatically). I then set the camera to record raw to the cf card and jpg to the sd card and repeated the test.

While I don't remember the exact figures (I did this about ayear ago shortly after I bought my 5D3) my second test was only able to capture about half of the images of the first test before the buffer filled and the shutter clicks slowed down. This seemed to indicate that if different image types are being written to the two cards the camera partitioned the buffer into spaces for both types and thus reduced the buffer size available to record only one type. Hence the smaller number of images able to be recorded before the buffer filled.

In your test you were recording one type of image to (first) the cf card and (second) both cards. Since the sd card is considerably slower than the cf card in image writes it only makes sense that, given a fixed time period, the time it took to write to the sd card would decrease the numer of images written. As I said, this seems consistent with how I understand the camera to work.

My primary concern is to not lose images so I use both cards and write raw to both at the same time. I can live with the added write time. Your needs are different and you have your camera set up accordingly. That is one of the things that makes the 5D3 a great camera. It is flexible enough to allow people with different needs to use it as best benefits them.